This invention relates generally to apparatus for treating the exhaust streams of lean-burn engines and, more particularly, to apparatus of this kind that catalytically oxidize hydrocarbons, particulate matter, and carbon monoxide, and that catalytically reduce nitrogen oxides.
Emissions Components
Lean-burn engines, such as diesel engines, emit various gases, including nitrogen oxides (NOx), gaseous hydrocarbons (HC), carbon monoxide (CO), and sulfur dioxide, SO2, and also emit combined solids and liquids called particulate matter (PM). This PM is composed of dry carbon (i.e., soot), inorganic oxides (primarily as sulfates) and liquids (such as condensed hydrocarbons). The sulfates, which are formed during combustion by the oxidation of sulfur present in the diesel fuel, react with the moisture in the exhaust to form sulfur acid, H2SO4. The liquids are a combination of unburned fuel and lubricating oils called the soluble organic fraction (SOF).
Treatment Needs
Diesel emissions are more complex than those from gasoline engines, and their catalytic treatment is more complicated. Governmental efforts to reduce engine emissions are becoming stronger, with concerns over global warming, human health effects, and high fuel prices leading to renewed interest in lean-burn engines. Intense focus has been given, in particular, to the reduction of PM and NOx in lean-burn engines. Most approaches aimed at reducing one of these two pollutants have led to an increase in the other. This characteristic, known as the PM/NOx trade-off, has remained problematic to the diesel engine industry. Therefore, these engines continue to require exhaust after treatment to meet these tougher emissions standards.
Current Treatment
Current oxidation catalysts used to reduce HC, CO and the soluble organic fraction of PM are typically washcoat-deposited on the channel walls of a cordierite- or another ceramic-based substrate. These catalysts are usually platinum- or palladium-based, and can be supported by alumina, zirconia, and vanadia with various promoter oxides such as rare earths. Current catalyst technology can reduce the soluble organic fraction of the PM as well as CO and HC, but cannot reduce NOx. Traditional automotive three-way catalysts cannot reduce NOx in excess air, such as is present in a lean-burn environment. A system solution is one way to achieve this simultaneous reduction of HC, CO, PM, and NOx.
The oxidation of HC and CO over a catalyst in a lean-burn environment has not traditionally been difficult, provided the exhaust gases and catalyst are above the so-called “light-off” temperature. Oxidation catalysts are typically composed of platinum or palladium impregnated on a washcoat that is applied to a catalyst substrate. For NOx reduction, a catalyst that could decompose NOx to N2 and O2 would provide the ideal solution; however, such a catalyst has proven difficult to develop. Alternatively, a large number of catalyst formulations have been shown to promote NOx reduction in the presence of HC. Using these formulations, the NOx acts as an oxidizer to convert HC to carbon dioxide and water. In practice, the HC reducing agent comes either from unburned fuel leaving the engine cylinder, or from post-combustion injection of fuel into the cylinder, exhaust manifold, or exhaust duct upstream of any exhaust aftertreatment apparatus.
While oxidation catalysts are effective in oxidizing the SOF component of the PM, they are not effective in oxidizing solid carbon. An alternative approach is to filter, or trap, the PM with a diesel particulate filter (DPF). However, DPFs have not been in widespread use, principally because the collected PM builds up on the filter, eventually leading to high back-pressure, which leads to reduced power and fuel economy. Further, excessive back-pressure from a blocked filter can damage the engine. A number of approaches have been proposed to burn off the PM, or to regenerate the DPF. These include, but are not limited to, the use of burners, catalytic fuel additives, and catalyzed DPFs.
There remains a need for an effective apparatus for reduction of all major pollutant types in a lean-burn environment, which is integrated and convenient, and which incorporates a NOx reduction function along with a DPF, and a means to regenerate the DPF. The present invention fulfills this need and provides further related advantages.